Nature of operations and summary of significant accounting policies (Joint Development Agreement) (Details) - Joint Development Agreement $ in Thousands	6 Months Ended
	Jun. 30, 2019 USD ($)	Sep. 25, 2017 USD ($) well
Joint Venture [Line Items]
Percentage of working interest in wells		15.00%
Percentage of working interest in wells upon achievement of required internal rate of return		75.00%
Joint venture cost overages recorded | $	$ 3,571
Canadian
Joint Venture [Line Items]
Number of joint venture stack wells | well		17
Garfield
Joint Venture [Line Items]
Number of joint venture stack wells | well		13
Bayou City Energy Management, LLC
Joint Venture [Line Items]
Funded percentage of drilling completion and equipment costs		100.00%
Number of joint venture stack wells | well		30
Percentage of working interest in wells		85.00%
Percentage of internal rate of return		14.00%
Percentage of working interest in wells upon achievement of required internal rate of return		25.00%
Bayou City Energy Management, LLC | Low
Joint Venture [Line Items]
Average well cost caps per gross well | $		$ 3,400
Bayou City Energy Management, LLC | High
Joint Venture [Line Items]
Average well cost caps per gross well | $		$ 4,000